You are here

Moon and Jupiter

An artist's concept shows the atmosphere of a planet known as HD209458b boiling away into space, as seen from a hypothetical moon. The planet and star are so close together that winds and radiation from the star are stripping away the planet's atmosphere. The star's energy and the planet's own magnetic field are also causing the planet to puff up like a big balloon. [NASA/ESA/G. Bacon (STScI)]

The biggest planet in the solar system puts in a big-time showing tonight. It rises not long after darkness falls, a little below the Moon. Jupiter looks like a brilliant star, so you won't have any trouble picking it out.

Jupiter is big enough to hold more than a thousand Earths. In fact, it's just about as big as a planet should get. Astronomers have discovered many planets in other star systems that are heavier than Jupiter, but that doesn't mean they're bigger. Their extra mass increases their gravity, which keeps them from getting wider.

But at least one planet defies the size limit. And one team of astronomers says it knows why.

The planet is HD209458b. It periodically passes in front of its star, blocking a bit of the star's light. How much the light dims, and how long the dimming lasts, reveals the planet's size -- about 30 percent wider than Jupiter.

The planet orbits close to its parent star, so it's quite hot. That puffs up its outer atmosphere a bit, but not nearly enough.

But a team from Caltech says the heat puffs up the star in another way. The combination of heat and strong winds gives an electrical charge to the upper layers of the atmosphere. A powerful magnetic field captures some of the charge and conducts it deep into the planet's interior. That heats up not just the outside, but the inside as well -- inflating HD209458b like a giant balloon.

Get Premium Audio

Listen to today's episode of StarDate on the web the same day it airs in high-quality streaming audio without any extra ads or announcements. Choose a $8 one-month pass, or listen every day for a year for just $30.